The invention relates to a filter for the decimation or interpolation of a digital signal.
Digital signal processing comprises several applications in which sampling should be changeable. In decimation, the sampling period is extended, whereby the sampling frequency decreases. This reduces the number of data points processed or stored in the memory per a time unit. In interpolation, the sampling period is shortened and the sampling frequency increases and the number of data points per a time unit increases.
There is a need to change sampling for instance in a radio system receiver in which an analogue-to-digital conversion is made to a pass band signal using a high sampling frequency. After the A/D conversion, the frequency of the pass band signal is decreased to a base band, but the sampling frequency still remains high. Sampling can be reduced by using a decimation filter, which also improves the signal-to-noise ratio.
An example of an ordinary decimating or interpolating filter, which changes the sampling of a signal by a high coefficient, is for instance a CIC (Cascaded Integrator-Comb) filter which comprises a set of integrators, a sampler and a set of comb filters after each other in a series. The solution is described in more detail in the publication: E. B. Hogenauer, An Economical Class of Digital Filters for Decimation and Interpolation, which is incorporated herein by reference.
A problem with the decimating and interpolating CIC filter is that the stop band does not efficiently attenuate the interfering signals. Especially if the stop band has interfering signals that are not exactly on the notch frequency of the CIC filter, the interference may damage the operation of the filter.
It is thus an object of the invention to provide an improved method and a filter implementing the method so as to efficiently attenuate interference on a wider frequency band. This is achieved by a filtering method in which a signal is processed digitally and sampling is changed by a coefficient M which is a positive integer and which defines alias frequencies on the frequency band of the filtering method. In the method, the signal is further filtered by at least one real Finite Impulse Response (FIR) filter having at least one stop frequency pair whose different frequencies are symmetrically on different sides of at least one alias frequency.
The invention also relates to a filter which is adapted to process a signal digitally and comprises a sampler for changing sampling by coefficient M which is a positive integer and which defines alias frequencies on the frequency band of the filtering method. Further, the filter comprises at least one real FIR filter having at least one complex stop frequency pair whose different frequencies are symmetrically on different sides of at least one alias frequency.
Preferred embodiments of the invention are set forth in the dependent claims.
The invention is based on using two stop frequencies, instead of just one, per each alias frequency, the stop frequencies being generated by a complex conjugate pair in a transfer function.
The solution of the invention provides several advantages. The stop band can be made wider, which enables a more efficient interference elimination and a more reliable filter operation.